Large quantities of chemical materials, such as acids, alkalis, salt, sulfides, lime, chrome tanning agents, fat-liquoring agents, and dyes, are used in the tanning process, a considerable part of which flow into the effluents. Tannery effluents mainly come from the preparation process, tanning process, and other wet-processing processes. The discharge of effluents from the soaking to dyeing procedures accounts for more than 90% of the total discharge of effluent from the whole tanning process. These effluents are a complex composition containing large quantities of poisonous and harmful substances, including lime, salts, fat, ammonia and nitrogen-based compounds, protein, sulfides, chromium salt, dyes, hair, hide trimmings, and dirt. The effluents contain very high Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) values. Due to the great variety of pollutants involved, the great range in the coefficient of variation for the quality and quantity of water, and their fetid smell, effluents from tanning factories are difficult to treat.
With the development of environmental protection equipment, the technology for treating tannery effluents has advanced (see Table 1). However, the cost of operating the treatment technology is still high and its efficiency is low. As a result, pollution from tannery factories is still not effectively controlled.
TABLE 1Current Technology for Treating Tannery EffluentTechnicalTechnologyTechnical AdvantageDeficiencyConventionalRemoval Rates:Requires a highactivated sludgeBOD5 > 90%quality effluent; isprocessCOD ≈ 60%-80%not applicable toChromaticity ≈ 50%-90%shock load; requiresS2− ≈ 85%-98%a large amount ofpower; is expensiveto install; requires alarge area; noteffective atremoving ammonianitrogenBiologicalRemoval Rates:Blockages are likelycontact oxidationBOD5 ≈ 95%and maintenance isprocessCOD ≈ 92%difficult; the entireS2− ≈ 98%system must bestopped whenperformingmaintenance.Oxidation ditchRemoval Rates:The treatment effectBOD5 > 95%is not stable; canS2 − ≈ 99%-100%only be used inSuspended solids ≈ 75%.warm weatherclimates.Bi-layer biofilterRemoval Rates:A suitable refluxBOD5 ≈ 98%ratio needs to beCOD ≈ 90%determined; suitableCr (III) > 96%filter materials areS2 ≈ 96%required; verySuspended solids ≈ 95%expensive.SBR processRemoval Rates:Very expensive; canCOD ≈ 93.3%only be deployed atS2− ≈ 99.0%a small scale; itsCr (III) ≈ 99.4%.efficiency is low;Suspended solids ≈ 90.3%maintenance isdifficult.
Currently, some companies recycle tannery effluents. However they only recycle effluents using a short procedure (see Table 1), which has a limited effect in reducing the discharge of effluents, the economy of chemical materials, and the reduction of production cost. It is a complicated and difficult problem to recycle all the effluents from all the procedures associated with the tanning process. Some of the problems associated with the above technologies include: the weight gain rate of limed pelt and the yield of wet-blue drops by 1.5%-3.5%, and the shrinkage temperature drops by 1.5° C.-3.5° C. Furthermore, the above processes will damage the grain side of the wet-blue, making the grain side rough and its color dark. This rougher surfaces results in the finished leather that fails to meet the requirements of some performance indexes.